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PROCESSING EQUIPMENT 73
film may increase, as it will on immersion in the processing solutions, and then decrease once more on drying. The shrinkage after drying is always sufficient to leave the final processed film shorter than it was on leaving the camera and before processing commenced. One simple method used to accommodate dimensional changes when using the rack type of processing unit is to anchor that end of the film which is at the centre of the rack, but to attach the outer convolution to the last pillar in use by means of an elastic band. This band is usually threaded through a hole punched in the film and then passed over the pillar. This provision is not wholly successful since it requires that each convolution of film shall move relative to its neighbours and by proportional amounts dependent upon the length involved in each turn. Whilst this movement may occur when the film is lengthening, that is, when it is in the solutions, it does not take place so well during the drying operation. Uneven shrinkage whilst drying causes severe strain to be set up in those sections of film which touch the supporting pillars of the rack assembly and may also cause noticeable drying marks.
The foregoing criticism is equally true when the film is transferred to a drum for the final drying operation. It is normal practice to spring-load the horizontal bars which form the drum to enable them to 'give' slightly towards the central supporting shaft. Even when this provision is made, and the film ends are attached to the drum by elastic loops, it is necessary to load the drum with great care and preferably to permit each turn in the spiral to remain noticeably slack on the drum if undue strain is to be avoided.
Many people are now familiar with the 'apron' type of tank, widely used to process still camera films and, particularly, those used in 35-mm 'Leica' and similar cameras. This process employs an ebonite central bobbin, usually split into two interlocking halves, fitted with perforated side flanges through which the processing solution may flow. Attached to the central bobbin is a length of relatively thick transparent celluloid bearing a series of raised hemispheres, moulded along each edge. The hemispheres are moulded in alternate directions, as seen in Figure 29, and serve to support the film between the convolutions of the apron whilst only permitting contact at the extreme edges and over very small areas. Tanks have been designed which permit a suitable key to be connected with the supporting bobbin and thereby permit the whole assembly to be rotated within the tank and during the processing operation. This undoubtedly helps to reduce the